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The propagation of an electromagnetic wave along a planar waveguiding structure that contains gaseous/solid-state plasma slabs with time-varying free carrier density is considered in the case when the time scale of the density variation is much greater than the wave period (adiabatic approximation). Both ionization and deionization (recombination, attachment) processes are included. General relations describing frequency shifting and energy losses of a guided wave are derived for an arbitrary open/closed waveguiding structure. Relations between the energy of the wave and its frequency conserved during the process of plasma density variation (adiabatic invariants) are found. Detailed analyses are given for surface waves guided by the boundary of a time-varying plasma half-space and a plasma slab. Slow variations of plasma density are shown to be energetically more efficient than fast variations. A new effect of reflection of the wave envelope that occurs without reflection of the fast oscillating carrier is pointed out.

An ultrafast optical pulse can create an electron-hole plasma in the depletion layer of a polar semiconductor, such as GaAs, and excite plasmon-phonon modes. We show that another optical pulse, which arrives later, always suppresses the excited plasmon-phonon modes and the suppression rate varies dramatically with the delay. The energy of the oscillations transfers to free-streaming currents which are not sustained by any electric field. The free-streaming currents consist of counter-directed streams of the carriers created by the first and second pulses. Our predictions are based on an analytical solution of the coupled equations for plasmonphonon modes with nonperturbative treatment of plasma density.Femtosecond optical pulses allow one to probe oscillations in solids, which occur on much longer-time scales, i.e., in the terahertz frequency range. Femtosecond pump-probe technique was used to detect oscillating changes in the optical properties in GaAs, 1 Ge, 2 and a number of other materials. 3,4 In 1995, Kuznetsov and Stanton 5 developed a microscopic theory of the plasmon-phonon oscillations generated by ultrafast optical excitation in polar semiconductors like GaAs. In polar semiconductors, coherent plasmonphonons are excited in the depletion region, where large built-in electric field exists. The optically created carriers move in the electric field and produce polarization. The polarization screens the field and induces a change in the lattice displacement. This results in the excitation of plasmonphonon modes.Recently, the dynamics of coherent plasmon-phonon modes in n-type GaAs at different doping levels was investigated by Cho et al. 6 It was found that both the background plasma and optically created plasma were involved in the coherent plasmon-phonon modes and the frequencies of the modes are determined by the total plasma density. The relaxation dynamics of the coherent phononlike LO-phononplasmon mode in n-GaAs were examined by Vallee et al., 7 and a large increase of the dephasing rate with electron density was found. Direct observation of the ultrafast decay of coherent plasmon-phonon oscillations in highly-doped n-GaAs was performed by Hase et al. 8 using a pump-probe technique. The generation of phonons by multiphoton optical absorption in GaN was also investigated. 9 Since the excitation of coherent plasmon-phonon modes is relatively well understood, 10,11 one may wonder about possibility of controlling their excitation with two ultrashort optical pulses that arrive with a delay. The investigation of such a possibility is our purpose in this paper. We show that the delay between the pulses affects the excitation of the plasmon-phonon modes to a significant degree. While this dependence may be expected, there are several features that distinguish the control of plasmon-phonon modes with two optical pulses from other schemes of coherent control. First, we show that the second optical pulse always suppresses the oscillations excited by the first pulse. This occurs because the total energy of the o...